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The use of milk progesterone measurements for genetic improvement of fertility traits in dairy cattle R F Veerkamp1, J K Oldenbroek1 and T van der Lende2 1Department of Animal Breeding and Genetics, ID-DLO P.O. Box 65, 8200 AB Lelystad, The Netherlands 2Animal Breeding and Genetics Group, WIAS, Wageningen Agricultural University, P.O. Box 338, 6700 AH Wageningen, The Netherlands. Abstract Fertility measures based on calving and insemination dates are heavily affected by a farmers decision to cull or inseminate a cow. This might partially explain the low heritability for these fertility traits and the unfavourable association between yield and fertility. Progesterone measurements might provide an alternative. Heritability estimates ranged from 0.14 to 0.20 (s.e. = 0.08) for interval between calving and first luteal activity (CLA). However, measuring this trait in a progeny testing scheme is not feasible. Therefore, mean progesterone profiles (transformed to a binomial scale) were investigated for groups of 50 animals differing in CLA. Using splines clearly different curves were observed, even when only monthly progesterone measurements were available for each cow. Days between calving till 50% of the daughters is cycling (progesterone > 3 ng/ml) might provide a good trait for sire selection. Phenotypic association between CLA and milk yield were unfavourable. If milk yield increased with 1 kg per day over the first 100 days of lactation, CLA is expected to increase with 1.4 days. Similarly, the regression coefficient of CLA on energy balance was -0.33 (s.e. = 0.11). Hence, at cow level progesterone measurements provide a heritable trait which is useful for selection and for investigating the relationship between fertility and production traits. Indications from this study are that progesterone measures during monthly milk recording might be useful to establish a mean progesterone profile for a group of animals. Whether this measure of female fertility is heritable and useful for sire selection in a progeny testing scheme requires a larger data set for further investigation. 1. Introduction
and many other factors. Error variances might be
relatively large therefore, rather than that genetic
Low fertility is of economic importance for dairy
variances for fertility traits are small. For example,
enterprises, because it results in higher levels of
compared with a genetic coefficient of variation of
involuntary replacement, slippage in calving pattern,
9.5% for milk yield, this parameter for calving
veterinary invention, hormonal treatment and interval, days to first service and conception to first reduced annual milk production (Esslemont and
service are 1.8%, 4.4% and 10.3%, respectively
Peeler, 1993). Even when the definition of optimal
(Price et al., 1997a). Further indications that errors
fertility performance might change (for example the
due to management decisions might affect
optimal length of the calving interval is still disputed
heritability estimates for fertility traits is that for the
for the highest productive cows) genetic interval to first heat a relatively high heritability of improvement of fertility remains important: it gives
0.15 has been reported. Furthermore, heritability
the farmer control over its fertility management (i.e.
estimates coming from records collected on an
which heat to inseminate, which month of the year
experimental farm, under controlled fertility
does he wants most milk, what is the optimal calving
management, are also relatively high, i.e. 0.08 for
interval). Hence, genetic improvement should give a
calving interval, 0.13 for days open and days to first
the farmer the option to inseminate cows at the
service (Pryce et al. 1997c). Hence, there appears to
economically most optimal time period, whatever the
be sufficient genetic variation for fertility traits, but
time period is. However, genetic selection for recording practises include too much noise with a improved fertility is hindered by the low heritability
for fertility traits (Jansen, 1985; Pryce et al. 1997b,
The second difficulty with using calving dates
and insemination dates to measure a cows fertility
One of the reasons for this low heritability might
objectively is that association between yield and
be that most fertility traits are based on calving and
fertility might be biased (Philipsson, 1981; Jansen,
insemination dates. These are obviously affected by
1985). Farmers might cull low producing cows
the farmers decision to inseminate or cull animals
earlier or delay inseminating a high producing cow
which both might have an affect on the observed
(Monday afternoon and Friday morning). Animals
correlation between yield and fertility.
with uterine infections or animals which had an
To overcome some of these problems, Darwash et
abortion were excluded from the data set. For all
al. (1997a, b) suggested the use of progesterone
other animals (n = 333) days to first luteal activity
profiles to measure the start of luteal activity. These
(CLA) was determined from the progesterone profile
authors demonstrated how early re-establishment of
as follows: when two consecutive samples were
ovarian activity (CLA), measured using progesterone
above 3 ng/ml the date of the first sample was taken
profiles, is an important prerequisite for high as start of luteal activity. Since progesterone samples fertility; one day delay in the interval to first ovarian
were taken till approximately day 100 in lactation,
activity was associated with 0.24 and 0.41 days extra
animals not cycling received a CLA of approx. 100
in the intervals to first service and conception,
respectively. Each additional 21 days in CLA On a subset of these animals (n = 226), data on progressively reduced the probability of failure at a
milk yield and energy balance during the first 100
given insemination to 0.89 of its previous value
days of lactation were available. More details of the
Darwash et al. (1997a). Furthermore, Darwash et al.
recording and data handling are given by Oldenbroek
(1997b) reported a heritability of between 0.13 and
0.28 for CLA. Hence, using progesterone profiles, a
measure of fertility is available which is unaffected
2.2 Analysis
by management decisions and seems to have a
The heritabilities (plus s.e.) were estimated using
Because only one estimate of the heritability for
VCE (Groenenveld, 1996). Additional to the random
CLA is available yet in the literature, the first
animal effect (including relationship matrix), year-
objective of this study was to estimate the season and age at calving were fitted as fixed effects. heritability of days between calving and first ovarian
Because its skew distribution, not only CLA was
activity measured using progesterone profiles.
analysed, but also two transformations of CLA: the
Measuring progesterone profiles on individual
cows is still costly and labour intensive as many milk
There were obviously not sufficient records to
samples (two or three times a week) need to be
investigate average profiles for individual bulls.
collected and analysed. This might only be feasible
However, to investigate differences in mean
on a small scale in for example a nucleus herd. As an
progesterone profiles for groups of around 50
alternative, using the mean progesterone profile of
animals, data were grouped according to CLA. Then,
many irregular sampled animals (for example average progesterone profiles were calculated for monthly milk recording) might be sufficient for a
each group separately, whereby the actual
progeny tested bulls. However, because not progesterone records were transformed in a binomial sufficient progesterone measurements were available
trait: al records above 3 ng/ml received the value of
to investigate average profiles for individual bulls,
1 and all other records the value 0. This latter trait
mean progesterone profiles for groups of around 50
has the advantage that it is easily interpretable
animals differing in CLA were investigated herein.
compared with actual progesterone levels, i.e. the
Finally, because CLA is unaffected by average indicates the proportion of animals with management decisions, the phenotypic association
progesterone > 3 ng/ml on a certain day (which
between CLA and both milk yield and energy
might be indicative for the proportion of animals
cycling at a certain date). Also the binary trait
simulates a relatively simple ‘yes or no’
2. Material and Methods
progesterone test. In practise, such a ‘dipstick test’
might have advantages when used on a large scale.
2.1. Data
To investigate mean profiles, curves were created
for each group separately. First, by simply averaging
Data were available for Holstein Friesian heifers
all the records per day of lactation, and secondly by
calving at the ID-DLO experimental farm ‘t Gen
fitting a smoothing spline in Genstat. The latter
from 1994 to 1996. Part of the animals in this
approach was applied on all records of individual
analysis are among the highest genetic merit animals,
cows (two a week; ca. 29 per animal), and on a
as these are part of the ‘Delta’ sib testing program.
subset of measurements for each cow (1 every
The other heifers originate from the ID-DLO farm
month; ca. 3 per cow) simulating measurements
and were on average about half a standard deviation
during normal milk recording. Hereto, each animal
below the Delta heifers for INET (the Dutch was assigned a random start day of first test (uniform production index).
distribution between 5 and 35 days), and
A data set was prepared containing two subsequently for each cow the records closest to four progesterone records per week for each animal
A simple linear regression was used to investigate
0.4 or 0.5 seem to be in the most distinctive part of
the phenotypic association between CLA and energy
The same conclusion can be drawn when only
monthly progesterone samples were available for
3. Results
each cow: it was still possible to identify the curves
for the six groups clearly (Figure 1c). Although the
3.1. Heritability for CLA
within group variation might be larger for sire
progeny groups than for our six groups, similar mean
In this study mean CLA was 38 days and the differences in CLA are expected between progeny standard deviation was 25 days. Heritabilities for
groups. This is because the genetic standard
CLA and its transformations ranged from 0.14 to
deviation for CLA is close to 11 days (Table 1).
0.20 (Table 1). A genetic standard deviation of 11
Therefore, it might be possible to estimate smoothed
days suggest that differences between animals are
mean progesterone profiles for bulls using
considerable for the interval between calving and
progesterone samples on daughters during normal
Table 1. Mean, standard deviation (in days) and 3.3. Milk yield and energy balance
heritability (s.e.) for days till first luteal activity
(CLA) based on progesterone profiles, and Another use of progesterone is that it measures loge(CLA) and 100/CLA (n = 333).
fertility more objectively, compared with relying on
h² insemination dates and recording of the farmer. For
this reasons it might provide a tool that enables us to
exclude the management effect (i.e. later
insemination of high producing cows) on the
relationship between yield and fertility. Figure 2
demonstrates that even when fertility is measured
3.2. Average group curves
using progesterone, the phenotypic association
between CLA and milk yield is unfavourable. When
Mean progesterone profiles for groups of 50 animals
daily milk increased with 1 kg a day over the first
with all twice weekly progesterone measurements
100 days, CLA is expected to increase with 1.4 days
animals are given in Figure 1a. Although curves are
(s.e. = 0.3). Similarly, the phenotypic relationship
clearly distinguishable between the groups, between
between energy balance and CLA is negative: the
days there is a lot of noise. This is not unexpected,
regression coefficient is -0.33 (s.e. = 0.11).
as, besides normal error variation, individual animals
were only sampled twice a week and curves of
4. Discussion
individual animals are not synchronised. Whilst a
normal cycling cows is expected to have one or two
The use of progesterone measurements for the
progesterone samples below 3 ng/ml every 21 days,
improvement of fertility records (as suggested by
and on average about 14 cows are measured each
Darwash et al. 1997a and 1997b) is demonstrated in
day, large sampling fluctuations in the mean this study. Using twice weekly samples on individual progesterone curves are expected. For the same
cows, days till first luteal activity could be
reason it is expected also that most curves asymptote
established. Heritabilities for CLA and its
around 0.7 - 0.8. Given that most of the animals in
transformations were not significantly different from
this study were not inseminated before day 100, it is
the values reported by these authors. Although, on
expected of normally cycling cows that, on a given
the measured scale our heritability was slightly
day, about 1/6 or 2/6 of the progesterone lower (although not significantly different from the measurements is below 3.0 ng/ml, because these
The mean value of 38 days is higher than the
When splines were fitted on all individual value of 27 days reported by Darwash et al. (1997b) progesterone records, but for each group separately,
and the standard deviation was nearly twice the
mean profiles became more distinct (Figure 1b).
value reported by these authors. There are several
Therefore it might be possible that from these
possible explanation for this: i) in our data set
smoothed curves, traits of interest for selection can
progesterone was measured twice a week only and
be derived. For example, interval between calving
started at day 15 only. Darwash et al. (1997b) had at
and the time period that 0.5 of the samples is above
least three progesterone measures a week. Given that
3.0 ng/ml seems to be an option to indicate fertility
CLA was based on two consecutive samples above 3
performance of a group of animals. The proportion
ng/ml, in our study we might have missed some initial small increases in progesterone; ii) animals
included in the study reported here were part of a
interval to post-partum ovulation and traditional
testing program and therefore hardly any animals
measures of fertility in dairy cattle. Animal
were culled before day 100. In commercial practice,
some animals might have been culled before they
Darwash, A.O., Lamming, G.E. and Woolliams, J.A.
show heat, and in fact not showing heat might be
1997b. Estimation of genetic variation in the
among the culling reasons; iii) mean milk yield in
interval from calving to postpartum ovulation of
our population was close to 30 kg/d, whereas the
dairy cows. Journal of Dairy Science 80:1227-
mean milk yield in the population reported by
Darwash et al. (1997a, b) was about 18 kg/d. If the
Esslemont, R.J. and Peeler, E.J. 1993. The scope for
regression of CLA on yield demonstrated a causal
raising margins in diary herds by improving
relationship (Figure 2), then CLA was expected to be
fertility and health. British veterinary Journal
However, the results in this study confirm that
Groenenveld, E. 1996. REML-VCE a multivariate
progesterone measures might provide a useful tool
multi model restricted maximum likelihood
for genetic selection. In herds where intensive
(co)variance component estimation package
progesterone sampling is practised (i.e. for
version 3.2. Users guide. Federal Research Centre
management reasons or in a nucleus herd) CLA can
be measured for individual animals. In the situation
Jansen, J. 1985. Genetic aspects of fertility in dairy
where only monthly milk recording is practised,
cattle based on analysis of AI data - a review with
mean progesterone profile based on a simple
emphasis on areas for further research. Livestock
‘dipstick test’ might by sufficient to distinguish
fertility performance of large progeny groups. Oldenbroek, J.K., Galesloot, P.A.J., Pool, M.H. and Splines can be used to smooth mean progesterone
van der Werf, J.H.J. 1997. Effects of selection for
profiles and based on these splines, the interval
milk yield on feed intake and metabolism of
between calving and days till 0.5 of the samples are
heifers in early lactation. Meeting of the EAAP
above 3.0 ng/ml provides a simple trait which could
be used as trait to distinguish differences between
Philipsson, J. 1981. Genetic aspects of female
groups. A larger data set with progesterone measured
fertility in dairy cattle. Livestock Production
is required, however, to establish whether the
heritability for this trait is as large as the heritability
Pryce, J.E., Nielsen, B.L., Veerkamp, R.F. and
Simm, G. 1997a. Genotype and feeding system
effects and interactions for health and fertility
Acknowledgements
Pryce, J.E., Simm G. and Veerkamp, R.F. 1997b.
Henry van der Gaast, Leo Kruijt and farm staff for
Genetics of fertility traits. Conference of the
assistance with the data collection and looking after
the animals. Holland Genetics for support towards
Pryce, J.E., Veerkamp, R.F., Thompson, R., Hill,
W.G. and Simm, G. 1997c. Genetic aspects of
References
fertility traits in Holstein Friesian dairy cattle (In
Darwash, A.O., Lamming, G.E. and Woolliams, J.A.
1997a. The phenotypic association between
> 3 0.6og. 0.5op.Pr 0.3Da ys in la cta tionog. > 3. 0.5op. pr 0.3Days in lactationog. > 3 0.5op prPr 0.3Da ys in la cta tionFigure 1. Mean progesterone profiles (prog >3.0 ng/ml sample is 1 otherwise sample is 0) for cows grouped according to their CLA. Median CLA values for the six groups are 17, 19, 24, 31, 47 and 85 days, respectively, and each group contains around 50 records. The top graph is the average of the raw data based on two progesterone samples a week per cow. In the second graph smoothing splines are fitted through this data. In the bottom graph monthly milk recording was simulated by using only one sample every 28 days per cow. Milk during first 100 days(kg/d)Energy balance (MJ /d)

Figure 2. Days till first luteal activity (CLA) measured using progesterone as a function of average a) milk yield and b) energy balance during the first 100 days of lactation